Process of heat treating ferromagnetic material



K. J. SIXTUS Sept. 26, 1961 PROCESS OF HEAT TREATING FERROMAGNETIC MATERIAL Filed NOV. 18, 1954 2 Sheets-Sheet 1 (Pb-8) SSflVS NI NOLLVZLLBNSVW OOOT OOON

80318830 NIBOHO! BAIOBBOO OISNIHlNI hZ EHZUF p 1961 K. J SIXTUS 3,001,943

PROCESS OF HEAT TREATING FERROMAGNETIC MATERIAL Filed Nov. 18, 1954 2 Sheets-Sheet 2 (I) (D D A B E z o C--2ooo 8 a E i i i i v O I 2 3 4 2000 000 x lo HELD OERSTEDSM ENERGY pnooucflan) (GAUSSXOERSTEDS) [17 V5 27 far M 40: (/VL/l/S S/xrus atent arm Patented Sept. 26, 1961 nice 3,001,943 PROCESS OF HEAT TREATING FERROMAGNETIC ATERIAL The present invention relates toa process for improving the magnetic properties of ferromagnetic materials, and to the improved materials thus produced.

Recently, an improved polyoxide material has been developed in which ferric oxide, Fe O has been combined in a mixed crystal with a secondary oxide such as barium oxide, strontium oxide or lead oxide. In some instances, small amounts of calcium oxide have been added as a partial replacement for one of the secondary oxides to improve certain magnetic properties.

This new polyoxide' type of material has a fairly high intrinsic coercive force, and a reasonably high residual induction. However, the maximum energy product, (BH)max, which represents the maximum product of the magnetic field and the induction determined on the demagnetization curve, is not always so high as might be desired. Maximum energy product is generally considered to be the best single criterion for the selection of materials as permanent magnets.

It is known that improved magnetic properties can be imparted to polyoxide materials by including an orientation step at a certain stage of the manufacture. After the mixed oxide powders have been sintered, they are milled to a degree of fineness at which each powder particle represents a single crystal. When such a powder is oriented and compacted in a strong magnetic field, the particles orient themselves with a certain crystallographic axis more or less parallel to the applied field. This preferred orientation persists after the sample has been sintered and imparts to the sample a high value of residual induction.

As indicative of what can be accomplished by orientation, polyoxides may be produced, using the orientation step, having a residual induction of 3500 gauss and a coercive force of 2000 oersteds, or the residual induction might be raised to as high as 3900 gauss with a decrease in coercive force to 1300 oersteds. In both cases the maximum useable energy would be found to be somewhat less than 3 l0 (gaussx'oersteds). Without the orientation step, but starting with the same composition of polyoxide and using. otherwise identical conditions of treatment, the magnetic properties would typically be a residual induction of around 2000 gauss, a coercive force of about 2000 oersteds and a maximum useable energy, (BI-I) max, of'about" 1 X10 (gaussx oersteds).

Thus, from past experience it appears that an increase in residual induction in'materials of this type is normally accompanied by a decrease in coercive force. For example, it has been found that raising the sintering temperature of these mixed oxide compositions, and of sintered magnets ingeneral, increases the residual induction while loweringthecoercive force. The first effect isapparently due to the increase in apparent density of the specimen, while the second seems to be associated with grain growth.

An object of the present invention isto provide an improved heat treatment method" for increasing the coercive force aud'fth'e maximum: useable magnetic energy of permanent magnet" material's consisting of a combination of iron oxideand one or. more metal oxides.

A further object" oftlie pres'ent' invention'i s to provide a process'for' making a permanent magnet material which is less susceptible to the demagnetizing influences of stray fields, shocks, and temperature changes.

A further object of the present invention is to provideimproved magnetic compositions characterized by a high cofrclve force and a high maximum useable energy prod uc Other and further important objects of this invention Will become apparent from the following description and appended claims, taken in conjunction with the accom panying drawings, in which:

FIGURE 1 is achart showing thevariations in intrinsic coercive force values, for given sintering temperatures, for varying periods of heat treatment given the samples in accordance with my' present invention.

FIGURE 2 isa chart showing in two curves the demagnetization portions of the hysteresis loop for two samples A and B, before andafter; respectively, being subjected to a heat treatment step of my invention.

FIGURE 3 is a chart showing similar curves for the same samples, that on the left hand side of the figure beinga plot of the induction against the applied mag-' neticforce, and that on the right hand side of the figure showing the curves resulting frommultiplying a given value of induction (3) by the corresponding value of magnetic field (H) to give-the energy product (BH').

In its broader aspects, the present invention involves increasing the maximum useable energy of a permanent magnet of the polyoxide type containing iron oxide and an oxide. of barium, strontium or lead.

Where only barium oxide, strontium oxide or lead oxide is combinedwith'iron oxide to form mixed poly-- oxide crystals; the-'crystals-are non-cubic. This isthe preferred typeof polyoxide to which my present method ofheat treatment is-applied. The group of these metals; barium, strontium andlead, willbe hereinafter referred was the primary group ofmetals.

In addition; however, some portion of any of these metals of the primary group may be replaced by metals of a secondary group. consisting of calcium, cobalt, chromium, aluminum and titanium. In general, the oxide of a secondary group metal should not be present in excess of 0.4 mole fraction of the primary'metal oxide present. If an oxide of a secondary group metal is present, the mixed metal oxide crystal: may or maynot be non-cubic. Calcium oxide is the preferred oxide of a secondary group metal, and, when present, is present in an amount notexceeding about' 0.4 mol. fraction of one of the primary" groupmetal oxides.

It has now been determinedthat by heating such compositions after sintering toanelevated temperature below the sintering temperature, and maintaining the composition in a mildly'oxidizingor inert atmosphere, min a non-reducing atmosphere, substantial increases in the maximum useable energy product of the permanent mag net material will result; As might be expected, the times and temperatures involved for treating the polyoxide compositions vary from. one oxide to another, but in general some beneficiation of the permanent magent material will result if the composition is heat treated for as little as ten minutes. In most instances, however, the period of time involved will be considerably longer. In a preferred embodiment of this invention, the treating time is maintained for at least four hours. From a theoretical standpoint, there seems to be no maximum time limit involved in the heat treating step, although it has been observed that after twenty hours in the heat treating furnace, little additional benefit is derived.

As previously mentioned, the temperature employed in the heat treating zone should be below the sintering temperature of the compositions involved, which for the type of composition here described is on the order of about 2500" F.. generally, between about 2300 and 2500 F. The best results obtained with the described process appear to occur from a heat treatment at a temperature in the range from 1500 to 2000 F.

The atmosphere in which the heat treatment takes place should be one incapable of effecting substantial reduction of the oxide materials present. In other words, a slightly oxidizing atmosphere such as air can be used to advantage, or an inert atmosphere such as nitrogen, carbon dioxide, helium, and the like may also be employed. Because of its convenience, an atmosphere of air will normally be preferred.

The initial steps of preparing the non-cubic mixed crystals of ferric oxide and the selected oxide or oxides are known and will be touched upon only briefly here. In general. the compositions are prepared by mixing the desired molecular proportions of materials (normally on the order of 6 molecular parts of Fe O for every molecular part of the primary group metal oxide).

The mixture of oxide powders is then formed into slugs and the slugs pre-fired at a temperature of between 2300 and 2500 F., usually around 2400 F. The temperatures and times of pre-firing should be such as to produce large crvstals. Lower temperatures within the range of 2300 to 2500 F. may be used if the starting crystals are small and active. and hi her temperatures within the same range may be used if the starting crystals are not active. The crystals formed as a result of the prefiring should be as large as 1-200 microns or larger in their greatest dimensions.

After being pre-fired. the slugs are crushed and ground, as by millin until the particles are of a micron or so in greatest dimension. These particles are then subjected to a strong magnetic field to etfect the orientation previously referred to, and compressed under high pressures, up to 3000 psi. and higher. into prismatic forms. The prismatic compacts are then passed into an oven and held at a sintering temperature appropriate for the material involved, and for a time sufficient to cause partial fusion at the grain boundaries of the article characteristic of a sintering process.

The process of the present invention may be combined with such a more or less conventional sintering process either by interrupting the normal slow cooling of the compact from sintering temperatures, or by heating the sample again to the heat treatment temperature after the sintered mass has cooled. Particularly good results have been obtained in the process of the present invention by cooling the sintered compact as it comes from the sintering furnace at a rate in excess of about 50 F. per minute down to room temperature, followed by heating to the heat treatment temperature, and again followed by a rapid cooling at a rate in excess of about 50 F. per minute.

The following specific examples illustrate the preparation of the magnetic compositions of the present invention.

EXAMPLE I Pro-fired slugs of a mixture of about 5.6 molecular parts Fe O and 1 molecular part BaO were ground until the resulting powder had a particle size such that each grain had a maximum linear dimension of the order of 1 micron and would essentially represent a single crystal. This powder was then pressed into prismatic specimens, using a pressure of about 3000 psi, while applying thereto a magnetic field of about 10,000 oersteds.

The pressed samples were then fired in three separate lots at the three different temperatures specified in Table I below. Three samples were cooled in the furnace from firing temperature to room temperature at a rate of about 200 F. per hour; two other samples were taken from the furnace and cooled from their respective firing temperatures in air at a rate of about 500 F. per minute. Table I shows the values of coercive force measured at INTRINSIC COERCIVE FORCE, H OF IRON OXIDE- BARIUM OXIDE MAGNETS Column I Column II Column III Column IV Column V With additional heat Sintered for 1 hr. at Cooled treatment at 1800' Firing Temp., F. in H Hours 11;;

0e. 0e. furnace... 1, 650 9 2, 085 furuace 1,130 9 1. 660 air 1,440 6 2, l20 furnace 975 1 1,200 air v 940 4 1,710

The above table shows in column V the value of coercive force obtained as a result of the additional heat treating step at 1800 F. for the respective periods of time given in column IV. All samples were cooled rapidly in air from the specified temperature. In all cases, a substantial increase in coercive force values resulted, varying between 225 and 770 oersteds. Since the residual magnetic induction, B and the shape of the demagnetization curve are affected only very slightly by the heat treatment, the maximum useable energy, BH- (max), was increased considerably.

EXAMPLE II The effect of the cooling rate was shown in the following test. Of four samples of the polyoxide material containing about 5 molecular proportions of Fe O for each molecular proportion of B-aO, each had a coercive force of about 1450 oersteds. All four samples were heated in air for three hours at a temperature of 1800 F. One of the samples was cooled at a rate in excess of 50 F. per minute from the furnace temperature of l800 to room temperature. The sample thus treated had a coercive force at room temperature of 1715 oersteds. The remaining samples were partially cooled in the furnace, one sample being cooled to 1200" 'F. in the furnace, followed by rapid cooling to room temperature; another was cooled to 800 F. in the furnace followed by rapid cooling to room temperature; and the third was cooled in the furnace to room temperature. All three of the last named samples had a coercive force value at room temperature only slightly greater than 1400 oersteds.

EXAMPLE H1 The effect of the time of heat treatment was determined in the following manner. Three samples were selected which had been sintered at 2300 F., 2360 F., and 2420 F., respectively. These samples were allowed to cool in the sintering furnace in each case until the temperature dropped to 1800 F. At this point the samples were removed from the furnace and were cooled in air. The three samples were then heat treated at 1800 F. for successively longer times, and the coercive force values were measured after each heat treatment stage.

The results of this experiment are illustrated graphically in FIGURE 1 of the drawings, in which the intrinsic coercive force, measured in oersteds, is plotted against the time of heat treatment, in hours, for each of three samples. It will be observed that in each case, the coercive force value tends to level off after about four hours and also that a higher sintering temperature results in a somewhat lower coercive force value being obtained.

The substantial difierences in the demagnetization curves which result when compositions are treated in accordance with the process of the present invention are illustrated graphically in FIGURE 2. In this drawing, the magnetization (B-H) is plotted for various values of magnetizing field. The curve labeled A represents the values obtained from a sample A of the polyoxide of Example I sintered at 2400 F. and cooled slowly in the furnace, according to established manufacturing practice. The curve B was obtained on a polyoxide material of identical chemical composition with sample A, but further subjected to a six hour heat treatment at a tem perature of 1800 F. in air.

in FIGURE 3, the left hand portion of the graph is a plot of the induction (B) against the applied magnetic field. The curve which results from multiplying a given value of induction (B) by its corresponding coercive force (H) value is shown on the right hand side of the plot. From these curves, it will be seen that the maximum energy product of sample A is substantially less than 3 X whereas the maximum energy product of the sample treated in accordance with the present invention is about 3.7x 10 In actual figures, it was found that the maximum useable energy dilference amounted to 32% in favor of sample B. As will be appreciated by those skilled in the at, this substantial increase in maximum useable energy enables the designer to build, for a given purpose, a magnet arrangement in a smaller space and having less weight than would be possible with a magnet of similar composition but manufactured according to present day practice.

The increase in coercive force achieved by the process of the present invention is accompanied by a significant increase in stability of the magnet against temperature changes. This was illustrated in the following experiment. A magnet substantially identical to sample A in the attached graphs and having the following dimensions: 0.335 inch in the flux direction and 0.402 and 0.883 inch perpendicularly thereto, had a coercive force of 1050 oersteds. After it was cooled to --60 F. and brought back to room temperature, the remanence had decreased by 45% of the starting value. The same experiment was carried out with the samples produced according to the present invention, and whose properties are illustrated in FIGURE 1 of the drawings. In each case it was found that the decrease inremanence for these samples after going through the same cooling cycle mounted to only 25 to 35% of the original.

From the foregoing, it will be appreciated that the process of the present invention provides a greatly improved magnetic composition characterized by a substantially increased maximum useable energy product. The process of the present invention has the further advantage that it requires no special apparatus to carry out the process as it can be readily carried out in existing heat treating equipment.

While the examples given are specific to a polyoxide of E2 0 and BaO, polyoxides of iron oxide and the oxide of any of the other primary group metals can be similarly prepared and subjected to the heat treatment step of my invention with improvement in maximum useable energy commensurate with the improvement reported in the case of iron oxide-barium oxide materials. In the case of the polyoxides formed from iron oxide, an oxide of one or" the primary metals and an oxide of one of the secondary metals, the process is essentially the same and improvement also results from the employment of my heat treatment step.

it will be evident that various modifications can be made to the described embodiments without departing from the scope of the present invention.

I claim as my invention:

1. The method of producing a permanent magnet having an increased intrinsic coercive force and maximum usable energy product which comprises sintering mixed non-cubic polyoxide crystals of ferric oxide and an oxide selected from the group consisting of the oxides of barium, strontium, and lead at a temperature in the range from about 2300 to 2500 F., rapidly cooling the sintered crystals in air at a rate in excess of 50 F. per minute, reheating the crystals to a temperature in the range from 1500 to 2000 F. in a non-reducing atmosphere for at least 10 minutes, and thereafter rapidly cooling the crystals in air at a rate in excess of 50 F. per minute.

2. The method of claim 1 in which said mixed polyoxide crystals are ferric oxide-barium oxide crystals.

3. The method of claim 1 in which said re-heating is carried out in an atmosphere of air.

4. The method of claim 1 in which said crystals are cooled both after sintering and after re-heating at a rate of about 500 F. per minute.

References Cited in the file of this patent UNITED STATES PATENTS Went et al Sept. 11, 1956 OTHER REFERENCES 

1. THE METHOD OF PRODUCING A PERMANENT MAGNET HAVING AN INCREASED INTRINSIC COERCIVE FORCE AND MAXIMUM USABLE ENERGY PRODUCT WHICH COMPRISES SINTERING MIXED NON-CUBIC POLYOXIDE CRYSTALS OF FERRIC OXIDE AND AN OXIDE SELECTED FROM THE GROUP CONSISTING OF THE OXIDES OF BARIUM, STRONTIUM, AND LEAD AT A TEMPERATURE IN THE RANGE FROM ABOUT 2300 TO 2500*F., RAPIDLY COOLING THE SINTERED CRYSTALS IN AIR AT A RATE IN EXCESS OF 50*F. PER MINUTE, REHEATING THE CRYSTALS TO A TEMPERATURE IN THE RANGE FROM 1500 TO 2000*F. IN A NON-REDUCING ATMOSPHERE FOR AT LEAST 10 MINUTES, AND THEREAFTER RAPIDLY COOLING THE CRYSTALS IN AIR AT A RATE IN EXCESS OF 50*F. PER MINUTE. 